It is important to know how to "take apart" a book. If it is a secondary source, I read the intro and conclusion (sometimes in reverse order). I look over the bibliography and look to see what kind of index and appendices it has. I then look to see if chapters have concluding paragraphs; whether or not, I read the first one, two or more paragraphs of each chapter followed by the corresponding few concluding paragraphs (or the concluding sections). This gives me an idea of what the author is arguing. Facts may dissolved, but you had better understand what the author is intending to say - that's something you should keep with you. And along those lines, I write a summary of the author's arguments. All that can happen before reading the whole thing. When pressed, I didn't always read the whole damn thing, but at least I was familiar with what the author intended, and that can be more powerful than trying (and inevitably failing) to commit a lot of facts to memory.

Primary sources are another matter. For these, I always read the editor/translator's introduction. In this century (I am writing from the perspective of a previous century), it is also possible to search the internet for summaries and discussions. With more ancient primary sources, this can be important because the standard of presenting the tale in literary fashion has changed a great deal, and some primary sources can be a hard nut to crack. I would use whatever tools I could lay my hands on to achieve that end - but if I were a student, I might not be forthcoming to a professor that I had used that approach. Not all approve of my suggestion!

I assume you are taking history because you like to read both primary and secondary sources, so ultimately, I also assume you really want to read as much as you can, but structuring your understanding of the sources before diving in may make your time spent more efficient. And if you run out of time, you will at least have a degree of familiarity with the assigned books even if you can't finish them all - if you begin by taking them apart.

I hope that helps.

Adding on to /u/itsallfolklore, the index is your friend.

If you've been assigned to write on a given topic, always check the index for related words/people/events/places. Often times my reading list for an essay would just point me towards a whole book (or four) but it turned out that I only needed maybe a single chapter or two, and that's covering background, the topic, and the aftermath/impact of the topic.

Also, regardless of what your reading assignment is, check to see if there's a good academic encyclopedia (not wikipedia!) that covers your areas of research and whether there's any articles on your topic. For many areas there will be a good Oxbridge guide to it (The Cambridge History of Iran, The Oxford Guide to Late Antiquity, The Cambridge History of Abassid Belles Lettres, etc. etc.) Even if it's not on the syllabus I don't think any professor will fault you for seeking more sources, and while it might sound like extra work having a good summarized grasp of a topic can save you a lot of time when you're actually doing research and deciding what's important or not.

Your other friend here (hopefully) is if your library's online catalogue, and its subscriptions to JSTOR and similar library subscription services. Those Cambridge histories and encyclopedias I mentioned before are almost always scanned online in JSTOR or something like it.

You can also search academic journals for reviews of the books you're reading! This can be exceptionally helpful, particularly if you're doing reading for essay writing, as the reviews will usually summarize the information in the book and then examine the author's arguments, and point you in the right direction for counter arguments.

I found that working your reading into other activities helps actually get it all done when you have a bulk. I read all of my soviet history readings in meal times.

Of course, this doesn't go without the help others have given, just a way to add in some more time to read if you need it.

I'll latch on to this part:

just too much for me to remember

Buy a pocket notebook, and take notes. Copy crucial quotes, summarize paragraphs, jot down interpretative notes, etc.

At my alma mater we didn't really get books to read past the first two years of the BA, after that it's all journal articles. When you're reading journal articles, start by only reading the abstract, introduction, conclusion and bibliography. The rest is filler.